The present invention relates to a halftone phase shift mask blank, a halftone phase shift mask, and a method of producing the same and, particularly, to a halftone phase shift mask blank suitable for use in a next-generation short-wavelength exposure light source such as an ArF excimer laser (193 nm) and an F2 excimer laser (157 nm).
For a dynamic random access memory (DRAM), mass production of 256 Mbit products has been established at present and a higher integration from a Mbit level to a Gbit level is making a progress. Following the development of higher integration, a design rule of an integrated circuit becomes finer. It is only a matter of time before a fine pattern with a line width (half pitch) of 0.10 μm or less is required.
As one approach to adapt for miniaturization of the pattern, a resolution of the pattern has been improved by shortening a wavelength of an exposure light source. As a result, a KrF excimer laser (248 nm) and an ArF excimer laser (193 nm) are mainly used as the exposure light source in the existing photolithography.
Although the shortened exposure wavelength improves the resolution, the depth of focus becomes shallow. This results in adverse influences, such as an increase in load imposed on a design of an optical system including a lens and decrease in stability of a process.
In order to solve the above-mentioned problems, a phase shift method has become used. In the phase shift method, a phase shift mask is used as a mask for transferring the fine pattern.
The phase shift mask comprises, for example, a phase shifter portion which forms a patterned portion on the mask, and an unpatterned portion in which the phase shifter portion does not exist. With this structure, light beams transmitted through both of the phase shifter portion and the unpatterned portions are shifted in phase by 180° with respect to each other to cause mutual interference of the light beams in a pattern boundary area. In this manner, contrast of a transferred image is improved.
It is known that a phase shift amount φ (rad) of the light beam passing through the phase shifter portion depends on a real part n of a complex refractive index and a film thickness d of the phase shifter portion and that the relationship given by the following equation (1) is established.φ=2πd(n−1)/λ  (1)
Here, λ denotes a wavelength of an exposure light beam. Therefore, in order to shift the phase by 180°, the film thickness d is given by:d=λ/{2(n−1)}  (2)The above-mentioned phase shift mask achieves an increase in depth of focus sufficient to obtain a desired resolution. It is therefore possible to improve both of the resolution and the applicability of the process without changing the exposure wavelength.
Practically, the phase shift mask is generally classified into a perfect transmission (Levenson type) phase shift mask and a halftone phase shift mask in accordance with a light transmission property of the phase shifter portion forming the mask pattern. In the former, the phase shifter portion has a light transmittance equivalent to that of the unpatterned portion (light transmitting portion). Thus, the former is a mask substantially transparent to the exposure wavelength and is generally effective in transfer of a line and space pattern.
On the other hand, in the latter, i.e., the halftone type, the phase shifter portion (light semi-transmitting portion) has a light transmittance on the order of about several percentages to several tens of percentages of that of the unpatterned portion (light transmitting portion). It is understood that this type is effective in preparation of a contact hole or an isolated pattern.
The halftone phase shift mask includes a double-layer type halftone phase shift mask comprising a layer for mainly adjusting the transmittance and another layer for mainly adjusting a phase; and a single-layer type halftone phase shift mask which is simple in structure and easy in manufacture.
At present, the single-layer type is a mainstream because it is easy in processing. In most cases, the halftone phase shifter portion comprises a single-layer film of MoSiN or MoSiON.
On the other hand, in the double-layer type, the halftone phase shifter portion comprises a combination of the layer for mainly controlling the transmittance and the layer for mainly controlling the phase shift amount. It is possible to independently control a spectral characteristic represented by the transmittance and the phase shift amount (phase angle).
On the other hand, with the miniaturization of an LSI pattern, it is expected that the wavelength of the exposure light source (exposure wavelength) will be shortened from the existing KrF excimer laser (248 nm) to the ArF excimer laser (193 nm) and further to the F2 excimer laser (157 nm) in future.
In the existing halftone phase shift mask, a film is typically designed so that the halftone phase shifter portion has an exposure light transmittance around 6%. However, in anticipation of a higher resolution, a higher transmittance is desired. In future, a transmittance of 15% or more will be required.
With the pursuit of the shortened wavelength of the exposure light source and the higher transmittance, the range of selection of a material of the halftone phase shifter portion which satisfies a predetermined transmittance and a predetermined phase shift amount tends to be narrowed. Moreover, with the increase in transmittance, a material having a high transmittance is required. Furthermore, with the shortening of the wavelength of the exposure light source, the material having a high transmittance is necessary as compared with the wavelength previously used. These requirements cause a problem that etching selectivity with a quartz substrate during patterning is reduced.
In the halftone phase shifter portion of a mutilayered structure comprising two or more layers, a phase difference and a transmittance can be controlled by a combination of multilayered or double-layer films. It is therefore easy to select the material. Furthermore, a material which serves as an etching stopper of an upper layer can be selected as a lower layer.
In the phase shift mask, a reflectance in the exposure light beam must be reduced to some extent. Generally, in a step of inspecting the appearance of the pattern, a light beam longer in wavelength than the exposure wavelength is used as an inspection wavelength and a transmission type defect inspection apparatus (e.g., KLA300 series, and the like) is used. Therefore, if the transmittance is excessively high (e.g., 40% or more) with respect to the inspection wavelength (for example, if the exposure wavelength is 248 nm (KrF excimer laser), the inspection wavelength is 488 nm or 364 nm), the inspection is difficult to perform.
Especially, with the shortened exposure wavelength, the halftone phase shifter portion having a high transmittance is required as described above. However, the material having a high transmittance tends to be large in increasing ratio of the transmittance with respect to the change in wavelength towards a longer wavelength. Therefore, in the single-layer halftone phase shifter, it is further difficult to reduce the transmittance with respect to the inspection wavelength to a predetermined range.
Furthermore, in the defect inspection apparatus, development of a new inspection method using transmitted and reflected light beams has been made. If the inspection is carried out in this method, the transmittance in the inspection wavelength may be slightly high (e.g., 50 to 60%) as compared with the inspection using the transmitted light. However, it is necessary to control the reflectance in the inspection wavelength so that the reflectance has some difference (e.g., 3% or more) from that of a transparent substrate.
Under the above-mentioned situation, the use of the halftone phase shifter portion of the multilayered type including two or more layers makes it easy to control reflection and transmission characteristics in the exposure and inspection light beams.
The double-layer halftone phase shift mask is described, for example, in Japanese Unexamined Patent Publication No. H4-136854 in which the halftone phase shifter portion has a double-layer structure comprising a thin Cr layer and a coating glass (Prior-Art Example 1).
It is known that the halftone phase shifter portion of a multilayered structure can be prepared by the use of a single common apparatus and a single common etchant for etching. For example, Japanese Unexamined Patent Publication No. H6-83034 discloses a mask including a halftone phase shifter portion having a multilayered structure in which the same element is contained in a plurality of layers (e.g., a double-layer structure comprising Si and SiN layers) (Prior-art Example 2).
Furthermore, the technique for reducing the transmittance with respect to the inspection wavelength is described in Japanese Unexamined Patent Publication No. H7-168343. Specifically, a double-layer structure includes a single-layer film known as a single-layer type halftone phase shifter, such as MoSiO or MoSiON, and a transmission film low in wavelength dependency of the transmittance in combination with the single-layer film. With this structure, a desired transmittance can be obtained with respect to both the exposure light beam (KrF excimer laser) and the inspection light beam (488 nm) (Prior-art Example 3).
Furthermore, a mask having the phase shifter portion of a multilayered structure using a tantalum-silicide-based material is described in Japanese Unexamined Patent Publication No. 2001-174973. Specifically, the halftone phase shifter portion has a double-layer structure which includes an upper layer containing tantalum, silicon, and oxygen as main components and a lower layer containing tantalum as a main component and not containing silicon (Prior-art Example 4).
Furthermore, Japanese Unexamined Patent Publication No. 2001-337436 discloses a mask including the halftone phase shifter portion having a double-layer structure which includes an upper layer containing tantalum, silicon, and oxygen as main components and a lower layer containing chromium and chromium-tantalum alloy as main components (Prior-art Example 5).
However, the above-described prior-art examples have the following problems.
Generally, a halftone phase shifter film is provided with a light shielding Cr layer formed thereon. The light shielding Cr layer serves as an etching mask layer for the halftone phase shifter film and is subsequently used to form a light shielding portion at a desired position on the mask.
In the structure of coating glass/thin Cr layer/glass substrate as in the Prior-art Example 1, the light shielding Cr layer is formed on the coating glass. In this event, preparation is made of a mask pattern of a three-layer structure of light shielding Cr layer/coating glass/thin Cr layer with a resist pattern, generally used in patterning, transferred thereon. Thereafter, the light shielding Cr layer is selectively removed typically by wet etching.
However, since the light shielding Cr layer is same in material with the thin Cr layer, there is a problem that the thin Cr layer is affected in the selective removing process of the light shielding Cr layer. Specifically, the thin Cr layer is etched and the pattern is sometimes completely removed in the principle similar to that of lift-off. When the thin Cr layer is side-etched, the transmittance in the vicinity of a pattern edge will inevitably be changed.
Next, in Prior-art Example 2, it is possible to continuously deposit, for example, the Si and the SiN layers by the use of the same sputtering apparatus and the same target of Si. However, if the SiN layer is deposited by reactive sputtering using the Si target and a sputtering atmosphere containing nitrogen, poisoning of the target by the reactive sputtering is caused to occur. As a result, reproducibility cannot be attained and productivity is degcreased. Furthermore, with the use of SiN, the transmittance becomes excessively low with the shortening of the exposure wavelength in recent years.
Next, in Prior-art Example 3, MoSiO or MoSiON is used as the material of the single-layer film (upper layer). However, by inclusion of the metal, the transmittance is reduced. Therefore, this example is not suitable for the recent shortening of the exposure wavelength. Moreover, if the content of the metal is reduced, the refractive index is reduced so that the film thickness of the halftone phase shifter increases. This is disadvantageous for the fine processing.
Furthermore, in Prior-art Examples 4 and 5, TaSiO is used as the material of the upper layer. However, by inclusion of the metal, the transmittance is reduced. Therefore, these examples are not suitable for the recent shortening of the exposure wavelength. Moreover, if the content of the metal is reduced, the refractive index is reduced so that the film thickness of the halftone phase shifter increases. This is disadvantageous for the fine processing.
Moreover, in these prior-art examples, the lower layer serves as an etching stopper against dry etching of the upper layer using a fluorine-based gas. Thereafter, the lower layer is etched by dry etching using a chlorine-based gas.
However, the lower layer containing tantalum in the Prior-art Example 4 has an insufficient etching selectivity with respect to the fluorine-based dry etching of the upper layer. In case of the chromium-tantalum alloy in the Prior-art Example 5, an etching rate with the chlorine-based gas is slow and a high-accuracy pattern is not obtained.